It is frequently assumed that the hard breakdown I-V characteristic in MOS devices with thin gate oxides can be represented by a linear relationship. However, the experimental data sometimes deviate significantly from this simple model, especially for severe breakdown events. At low voltages the current typically follows a diode-like behaviour, whereas at the largest biases the current is dominated by a series resistance effect. In the case of the non-linear post-breakdown conduction mode this resistance is not constant. In this work, an analytical expression for the non-linear I-V characteristic using the generalized diode equation in conjunction with the transmission properties of nano-sized constrictions is proposed. © 2008 Elsevier Ltd. All rights reserved.